For various reasons it is often desirable to provide vehicles capable of operation on conventional roadways and also on rail tracks. Such vehicles are used primarily in railway maintenance.
It is known to adapt conventional trucks and tractor trailers for use on rails, these vehicles being provided with retractable guidewheels which maintain the position of the vehicles on the railway tracks and which may be retracted for normal road use of the vehicle. To adapt a road truck for use on rails, a set of railwheels must be provided at the front of the truck, to act as guides and also to support the weight normally borne by the front wheels which are wider spaced than the gauge of conventional rail tracks. A rear set of railwheels must also be provided, but these wheels typically only support a small portion of the weight normally supported by the rear roadwheels; for the majority of conventional gauge railtracks and conventional trucks, the inner wheels of double sets of roadwheels rest on the rails, and are used to provide drive for the truck. Typically, a truck of this type will be provided with a crane boom and rail car couplings to allow a rail car to be pulled behind the truck.
In dry weather, such vehicles perform adequately and are capable of pulling one or more loaded rail cars, the rubber to steel contact between the driven rear tires and the rails providing sufficient traction and braking to allow the truck to tow the cars. However, in wet or icy conditions, the traction available decreases considerably and is often insufficient to allow the truck to tow and safely brake railway cars, or even for the truck itself to negotiate an incline.